1. Field of the Invention
The present invention relates to a method and arrangement for the traversing or passing-through of the critical rotational speeds of elongate rotors.
Supercritically running machines, whose rotors must traverse their flexually critical rotational speeds in order to reach their operative rotational speeds are becoming of increasing significance in the technology. To be mentioned as examples thereof are helicopter shafts, multi-stage gas-steam turbines, as well as other hollow and solid elongate rotating bodies. Usually, such types of rotors must be balanced as well as possible and forced through their critical speeds with increased drive power.
It is known to change the gripping conditions at the bearing support locations so as to provide for short-term variation of the frequencies of the flexural resonances, and to thereby avoid large resonance amplitudes, this being valid for the lowest (basic oscillation) and also their higher oscillations. The change in the gripping or support conditions must be effected at the correct point in time so that, through alternating gripping or release of the rotor during acceleration or deceleration, there are never reached dangerous amplitudes which could bend or even destroy the rotor in an extreme case. The stable rated operating drive is then obtained intermediate two resonance frequencies.
2. Discussion of the Prior Art
A method is known in which the fastening or loosening of the rotor ends is effectuated through the intermediary of controllable bearings or supports. For this purpose there is employed the physical effect in that the frequency of the critical rotational speed for a rotor whose ends are only loosely restrained (gripping condition: "free", "free") lies higher as compared to when one end or both ends are easily rotatable about its longitudinal axis but supported against radial movements. However, this known method, for the control of the correct time point for switching from "free" to "supported" and reversely requires a considerable complex technical construction, so as to be rarely applied in the technology.
A similar effect as that in the change of the restraining conditions at the rotor ends can be achieved through alternating gripping conditions also at the present nodes of the oscillation of the rotor. These arrangements can influence the rotor from externally thereof, for example, by means of contactable rollers, variable electrical or magnetic fields, air forces, gas or liquid forces, or also from the interior of a hollow rotor by means of suitable construction elements. In this instance there also appear the mentioned technical difficulties, when the change must be effected at the correct point in time.